1. Field of the Invention
The present invention relates to a multi-wavelength channel transmission filter, and particularly, to an improved multi-wavelength channel transmission filter which is capable of stabilizing and standardizing wavelengths (or frequencies) of multi-wavelength (or multi-frequency) channels simultaneously for a wavelength (or a frequency) division multiplexed high capacity optical communication system, and separating multiple wavelength channel signals, and is well adaptable to various application and practical uses.
2. Description of the Conventional Art
In the conventional wavelength (or frequency) division multiplexed large capacity optical communication systems, it is necessary to stabilize and standardize the wavelengths (or the frequencies) of multiple wavelength (or frequency) channels, and it is important to secure a device capable of separating multiple wavelength channel signals.
In addition, in the high density wavelength-division-multiplexed optical communication systems, a WDM channel scheme with unequal channel spacings is necessary to minimize the degradation or interference between channels due to optical nonlinear effects. (IEEE Photonics Technology Letters, Vol. 6, pp. 754-756, 1994)
In this optical communication system with multiple wavelength channels of unequal channel spacings, a method of using a number of the conventional single wavelength optical filters may be used. However, this method is not effective for a system composition because the optical wavelength filters are bulky, and may not be possibly used as a simultaneously transmitting multi-wavelength filer of low optical power loss.
Various types of optical filters have been studied. Among the optical filters, there are optical filters based on a bulky optics pigtailed with optical fibers and on optical fiber Bragg grating filters. The optical fiber Bragg grating filter is fabricated by exposing a ultraviolet (UV) ray laser beam to a UV sensitive optical fiber, and acts as a single wavelength reflection filter.
Among them there are a few type of optical filters capable of simultaneously separating and transmitting multiple wavelength (or frequency) channels. One of them is interferometer type optical filters which are formed as arrayed waveguide gratings arranged on a silicon wafer. Another type is an optical filter formed with a bulky grating optics and many optical fibers (or optical waveguides). In the latter type of filter, the reflected (or transmitted) beams from a reflection (or transmission) type bulk grating optics are launched into many optical fibers (or optical waveguides), each of which takes out a single wavelength channel. In addition, this type of grating is very difficult to fabricate since the position of the optical fiber or the optical waveguide, in which each wavelength channel diffracted by the diffraction grating is selected, is critical to accurate wavelength selections and sensitive to environment. This filter has a function of separating individual wavelength channel signal from the input multiple wavelength (or frequency) channel signals into each optical fiber or optical waveguide, but does not have a function of simultaneously separating a selectable choice of the multiple wavelength channels into one optical fiber or waveguide.
Technology for the interferometer-type multiple wavelength silica waveguide filter has been improved these days. However, disadvantages of this technology are its limited filtering mechanism, which filters out multi-wavelength channels separated only at a constant wavelength (or frequency) interval, and difficulty of selective filtering of arbitrarily separated multi-wavelength channels.
FIGS. 1 and 2 are views illustrating the construction of optical filters capable of separating multi-wavelength signals simultaneously in conventional arts.
FIG. 2 illustrates a multi-wavelength transmitting filter formed a number of pairs of identical Bragg gratings which have a different wavelength response for each pair and are connected to a polarization-division-multiplexer (PDM) through polarization controllers. For this type of filter the identity of the grating pair is very critical, and fiber lengths from the PDM to each grating of the same pair should be kept the same, which are not practically easy to use. In addition, for this type of filter there exist optical time delays among) the reflected signals of different wavelengths due to different travel distances between the PMD and each pair of the Bragg gratings. (S. V.
Chernikov and J. R. Yaylor, Optics Letters, Vol. 20, No. 14, pp. 1586-1588, 1995)
FIG. 2 illustrates a multi-wavelength transmitting filter formed with an optical circulator and a number of Bragg gratings of different wavelength responses. For this type of filter there also exist phase delays among the different wavelength channels due to the different locations of the Bragg filters from the optical circulator. Since the inherent group velocity delay between different wavelength channels in the waveguide's chromatic dispersion may (or can) be compensated with recent technologies on dispersion compensation in a relatively easy way, the transmission time delay among the different wavelength channels may cause a problem or require additional temporal delay circuit(s) in all-optical wavelength conversion and time synchronization between channels for high-capacity optical transmission networks. (Japan Patent No. JP07336327 A 951222 (9609) H04J-014/00)